1. Field of the Invention
The present invention relates to a transfer device mounted in an image forming apparatus, such as a copying machine, a printer, or a facsimile apparatus, which uses an electrophotographic technology.
2. Description of the Related Art
Currently, a single-drum type image forming apparatus and a tandem type image forming apparatus are known. The single-drum type image forming apparatus includes a single image bearing member (i.e., photoreceptor drum), whereas the tandem type image forming apparatus includes two or more image bearing members. Since the tandem type image forming apparatus provides high productivity in comparison with the single-drum type image forming apparatus, the tandem type has become widespread in recent years.
Examples of the tandem type image forming apparatus include one that directly transfers a toner image from an image bearing member onto a transfer material on a transfer conveying body, and one that first transfers a toner image from an image bearing member onto an intermediate transfer body and finally transfers the toner image onto the transfer material. Each type is configured such that some of the image bearing members can be separated from the transfer conveying body or intermediate transfer body respectively. This is because constant contact of each image bearing member with the transfer conveying body or intermediate transfer body may lead to scratching or wear after long use and, therefore, it is necessary that image bearing members for colors not involved in an image formation be separated from the transfer body or the like in order to prolong the life of the components. Generally, such a separation is switched between a monochrome mode and full-color mode. For example, in the case of an apparatus in which four image stations are arranged in tandem, image forming units for the four generally-used colors (black, cyan, magenta, and yellow) are designed such that only the image bearing member for black and the transfer body are brought into contact with each other in monochrome mode whereas image bearing members for all the colors and the transfer body are brought into contact with each other in full-color mode (refer to Japanese Patent Application Laid-Open No. H9-146383).
Meanwhile, such a tandem type image forming apparatus suffers from image defects caused by banding. Banding occurs when a toner image is blurred or a dot image is distorted due to unstable rotating speed of the transfer body or an image bearing member during the formation of toner images on the image bearing members or during the transfer of toners from the image bearing members to the transfer body.
The causes of banding with respect to components involved in the image transfer reside in unstable rotating speed of the transfer body. Factors contributing to unstable rotating speed of the transfer body may be shock from starting or stopping of a rotating body provided in the apparatus, shock from the operation of a contact/separation member, shock from a transfer material, such as paper, entering or exiting the transfer body, etc.
In order to prevent image defects caused by banding, a conventional technique increases electrostatic attraction between an image bearing member and the transfer body, thereby making the speed of the transfer body less susceptible to disturbance.
An example of such a technique is described in Japanese Patent Application Laid-Open No.2005-128230. The application discloses an image forming apparatus including: a latent image forming unit that forms an electrostatic latent image on an image bearing member; a development unit that develops the electrostatic latent image into a toner image; a primary transfer unit that transfers the toner image to an intermediate transfer body; and a secondary transfer unit that transfers the toner image, transferred to the intermediate transfer body, to a recording medium, and the apparatus includes a member for a contact/separation operation or driving/stopping operation, which is performed in the non-image formation area on the intermediate transfer body or in an area of the image bearing member, which area corresponds to the non-image formation area. This image forming apparatus sets primary transfer bias to high while the position on the intermediate transfer body, where the separation/contact operation or driving/stopping operation is performed, is passing through a primary transfer position.
However, the inventors of the present invention discovered that such countermeasures are not enough to prevent image defects caused by banding, and examined the factors contributing to banding in detail. It has been found that more defects arose while the image bearing members and the transfer body are partially separated from each other. Further analysis revealed that a decrease in electrostatic attraction in the area where the image bearing member and the transfer body are separated results in a decrease in the overall electrostatic attraction between the image bearing members and the transfer body. This increases instability of the transfer body.
In order to avoid such defects, transfer bias is further increased using conventional techniques. This reduces banding. However, the excessively high transfer bias results in inconveniences such as formation of an abnormal discharge image, a reverse transfer image, etc., and thus desirable image quality cannot be obtained.